This invention relates to separating and removing suspended particles from molten metal by passing the molten metal containing the suspended particles preferably generally upwardly through a porous media. The particles may be either liquid particles or solid inclusions. The porous media is constructed and arranged such that the movement of the molten metal therethrough renders the suspended particles gravity separable.
Molten metal, such as aluminum, including alloys containing over 50% aluminum, has been treated to remove impurities therefrom. Some of these treatments, such as in-furnace fluxing or in-line treatment, generate liquid particles, such as molten salts. Also, additions of salts are often made to furnaces to reduce melt loss. A portion of these molten salts are carried along with the molten metal and, if not removed from the molten metal, can create "oxide patches" on the surface of the solidifying ingot. These oxide patches not only adversely affect ingot quality but also many times have to be scalped off the ingot, which results in a reduction of the recovery of metal in the ingot casting operation. Oxide patches can also cause ingots to crack. In some cases, the oxide patches are so prevalent that the entire ingot must be scrapped and remelted. This, of course, adds cost to the ingot casting operation.
The liquid particles referred to, such as the molten salts, are in the micron size range, typically from less than 1 micron up to 80 or possibly 100 microns in size. The molten salts are usually MgCl.sub.2, NaCl, CaCl.sub.2, KCl, LiCl and mixtures thereof. The molten salts can also contain NaF, AlF.sub.3 and CaF.sub.2 originating from furnace additives or potroom metal. Most liquid salts are lighter than the molten metal and would be gravity separable but remain entrained or suspended largely because their small size results in an extremely slow rise velocity. Other than for their small size, the buoyant liquid particles would rise to the surface for removal by skimming or similar operations.
There have been efforts to remove particles, such as solid and liquid inclusions, from molten metal. U.S. Pat. No. 4,390,364 discloses a removal method comprising moving the molten metal containing suspended particles downwardly through a medium of submerged contacting surfaces such as a packed bed. The contacting medium has a high void fraction and a high specific surface area. The patent states that this method results in coalescence or agglomeration of the suspended particles. Afterwards, the molten metal is passed generally downwardly through inclined channels or passages. Buoyant particles and agglomerates collect on the underside of the inclined surfaces and typically move upward in a counter-flow relationship with the metal, where the metal moves laterally and downwardly through the inclined passages. These buoyant particles and agglomerates are removed by skimming or similar operations.
U.S. Pat. No. 4,790,873 discloses a method of removing particles from molten metal by contacting the molten metal with a medium which retains metal-nonwettable inclusions and then passing the molten metal through a filter of metal-wettable material. The metal-wettable material of the filter retains the liquid particles therein. The filter containing the liquid particles is subsequently removed and replaced.
Despite these known methods and apparatus, there remains a need for an improved method for separation and removal of liquid particles from molten metal.
It is an object of the invention to remove particles from molten metal before casting of the molten metal into ingots.
It is a further object of the invention to provide a method and apparatus which creates gravity settlable particles from particles suspended in the molten metal.
It is yet another object of the invention to provide a porous media which is constructed and arranged so as to facilitate coalescence of particles therein for subsequent carrying out by the molten metal and ultimate removal from the molten metal surface by skimming or a like operation.
It is still another object of the invention to provide a liquid particle removal method and apparatus that will result in increasing the useful lifetime of the bed filter and/or other filtration systems used in the casting process.